Ioannis Contopoulos

The Axisymmetric Pulsar Magnetosphere

We present, for the first time, the structure of the axisymmetric force-free magnetosphere of an aligned rotating magnetic dipole, in the case in which there exists a sufficiently large charge density (whose origin we do not question) to satisfy the ideal MHD condition, E B=0, everywhere. The unique distribution of electric current along the open magnetic field lines that is required for the solution to be continuous and smooth is obtained numerically. With the geometry of the field lines thus determined, we compute the dynamics of the associated MHD wind. The main result is that the relativistic outflow contained in the magnetosphere is not accelerated to the extremely relativistic energies required for the flow to generate gamma rays. We expect that our solution will be useful as the starting point for detailed studies of pulsar magnetospheres under more general conditions, namely, when the force-free and/or the ideal MHD condition, E B=0, are not valid in the entire magnetosphere. Based on our solution, we consider that the most likely positions of such an occurrence are the polar cap, the crossings of the zero space charge surface by open field lines, and the return current boundary, but not the light cylinder.

Three-dimensional numerical simulations of the pulsar magnetosphere: preliminary results

We investigate the three-dimensional structure of the pulsar magnetosphere through time-dependent numerical simulations of a magnetic dipole that is set in rotation. We developed our own Eulerian finite difference time domain numerical solver of force-free electrodynamics and implemented the technique of non-reflecting and absorbing outer boundaries. This allows us to run our simulations for many stellar rotations, and thus claim with confidence that we have reached a steady state. A quasi-stationary corotating pattern is established, in agreement with previous numerical solutions. We discuss the prospects of our code for future high-resolution investigations of dissipation, particle acceleration, and temporal variability.

Revised Pulsar Spin-down

We address the issue of electromagnetic pulsar spin-down by combining our experience from the two limiting idealized cases that have been studied thoroughly in the past: that of an aligned rotator, in which ideal MHD conditions apply, and that of a misaligned rotator in vacuum. We construct a spin-down formula that takes into account the misalignment of the magnetic and rotation axes, and the magnetospheric particle acceleration gaps. We show that near the death line, aligned rotators spin down more slowly than orthogonal ones. In order to test this approach, we use a simple Monte Carlo method to simulate the evolution of pulsars and find a good fit to the observed pulsar distribution in the P- diagram without invoking magnetic field decay. Our model may also account for individual pulsars spinning down with braking index n 3, and that the older pulsar population has preferentially smaller magnetic inclination angles. We discuss possible signatures of such alignment in the existing pulsar data.

MAGNETOHYDRODYNAMIC ACCRETION DISK WINDS AS X-RAY ABSORBERS IN ACTIVE GALACTIC NUCLEI

We present the two-dimensional (2D) ionization structure of self-similar magnetohydrodynamic (MHD) winds off accretion disks around irradiated by a central X-ray point source. Based on earlier observational clues and theoretical arguments, we focus our attention on a subset of these winds, namely those with radial density dependence n(r) ∝ 1/r (r is the spherical radial coordinate). We employ the photoionization code XSTAR to compute the ionic abundances of a large number of ions of different elements and then compile their line-of-sight (LOS) absorption columns. We focus our attention on the distribution of the column density of the various ions as a function of the ionization parameter ξ (or equivalently r) and the angle θ. Particular attention is paid to the absorption measure distribution (AMD), namely their Hydrogen-equivalent column per logarithmic ξ interval, dNH/d log ξ, which provides a measure of the winds’ radial density profiles. For the chosen density profile n(r) ∝ 1/r the AMD is found to be independent of ξ, in good agreement with its behavior inferred from the X-ray spectra of several active galactic nuclei (AGNs). For the specific wind structure and X-ray spectrum we also compute detailed absorption line profiles for a number of ions to obtain their LOS velocities, v ∼ 100−300 km s (at log ξ ∼ 2−3) for Fexvii and v ∼ 1, 000 − 4, 000 km s (at log ξ ∼ 4 − 5) for Fexxv, in good agreement with the observation. Our models describe the X-ray absorption properties of these winds with only two parameters, namely the mass-accretion rate ṁ and LOS angle θ. The probability of obscuration of the X-ray ionizing source in these winds decreases with increasing ṁ and increases steeply with the LOS inclination angle θ. As such, we concur with previous authors that these Email: Keigo.Fukumura@nasa.gov University of Maryland, Baltimore County (UMBC/CRESST), Baltimore, MD 21250 Astrophysics Science Division, NASA/Goddard Space Flight Center, Greenbelt, MD 20771 Research Center for Astronomy, Academy of Athens, Athens 11527, Greece Department of Physics, Technion, Haifa 32000, Israel Senior NPP Fellow

The coughing pulsar magnetosphere

Polar magnetospheric gaps consume a fraction of the electric potential that develops across open field lines. This effect modifies significantly the structure of the axisymmetric pulsar magnetosphere. We present numerical steady-state solutions for various values of the gap potential. We show that a charge starved magnetosphere contains significantly less electric current than one with freely available electric charges. As a result, electromagnetic neutron star braking becomes inefficient. We argue that the magnetosphere may spontaneously rearrange itself to a lower energy configuration through a dramatic release of electromagnetic field energy and magnetic flux. Our results might be relevant in understanding the recent December 27, 2004 burst observed in SGR 1806-20.

A Cosmic Battery

We show that the Poynting-Robertson drag effect in an optically thin advection-dominated accretion flow around active gravitating objects generates strong azimuthal electric currents that give rise to astrophysically significant magnetic fields. Although the mechanism is most effective in accreting compact objects, it also seems very promising as a way to account for the generation of stellar dipolar fields during the late protostellar collapse phase, when the star approaches the main sequence.

Towards resolving the crab sigma-problem: a linear accelerator?

Using the exact solution of the axisymmetric pulsar magnetosphere derived in a previous publication and the conservation laws of the associated MHD flow, we show that the Lorentz factor of the outflowing plasma increases linearly with distance from the light cylinder. Therefore, the ratio of the Poynting to particle energy flux, generically referred to as σ, decreases inversely proportional to distance from a large value (typically 104) near the light cylinder to σ 1 at a transition distance Rtrans. Beyond this distance, the inertial effects of the outflowing plasma become important, and the magnetic field geometry must deviate from the almost monopolar form it attains between Rlc and Rtrans. We anticipate that this is achieved by collimation of the poloidal field lines toward the rotation axis, ensuring that the magnetic field pressure in the equatorial region will fall off faster than 1/R2 (R being the cylindrical radius). This leads both to a value σ = σs 1 at the nebular reverse shock at distance Rs (Rs Rtrans) and to a component of the flow perpendicular to the equatorial component, as required by observation. The presence of the strong shock at R = Rs allows for the efficient conversion of kinetic energy into radiation. We speculate that the Crab pulsar is unique in requiring σs 3 × 10-3 because of its small translational velocity, which allows for the shock distance Rs to grow to values Rtrans.

THE INVARIANT TWIST OF MAGNETIC FIELDS IN THE RELATIVISTIC JETS OF ACTIVE GALACTIC NUCLEI

The origin of cosmic magnetic (B) fields remains an open question. It is generally believed that very weak primordial B fields are amplified by dynamo processes, but it appears unlikely that the amplification proceeds fast enough to account for the fields presently observed in galaxies and galaxy clusters. In an alternative scenario, cosmic B fields are generated near the inner edges of accretion disks in active galactic nuclei (AGNs) by azimuthal electric currents due to the difference between the plasma electron and ion velocities that arises when the electrons are retarded by interactions with photons. While dynamo processes show no preference for the polarity of the (presumably random) seed field that they amplify, this alternative mechanism uniquely relates the polarity of the poloidal B field to the angular velocity of the accretion disk, resulting in a unique direction for the toroidal B field induced by disk rotation. Observations of the toroidal fields of 29 AGN jets revealed by parsec-scale Faraday rotation measurements show a clear asymmetry that is consistent with this model, with the probability that this asymmetry came about by chance being less than 1%. This lends support to the hypothesis that the universe is seeded by B fields that are generated in AGNs via this mechanism and subsequently injected into intergalactic space by the jet outflows.

GAMMA-RAY LIGHT CURVES FROM PULSAR MAGNETOSPHERES WITH FINITE CONDUCTIVITY

We investigate the shapes of γ-ray pulsar light curves using three-dimensional pulsar magnetosphere models of finite conductivity. These models, covering the entire spectrum of solutions between vacuum and force-free magnetospheres, for the first time afford mapping the GeV emission of more realistic, dissipative pulsar magnetospheres. To this end we generate model light curves following two different approaches: (1) We employ the emission patterns of the slot and outer gap models in the field geometries of magnetospheres with different conductivity σ. (2) We define realistic trajectories of radiating particles in magnetospheres of different σ and compute their Lorentz factor under the influence of magnetospheric electric fields and curvature radiation-reaction; with these at hand we then calculate the emitted radiation intensity. The light curves resulting from these prescriptions are quite sensitive to the value of σ, especially in the second approach. While still not self-consistent, these results are a step forward in understanding the physics of pulsar γ-radiation.

The Extended Pulsar Magnetosphere

We present the structure of the 3D ideal MHD pulsar magnetosphere to a radius ten times that of the light cylinder, a distance about an order of magnitude larger than any previous such numerical treatment. Its overall structure exhibits a stable, smooth, well-defined undulating current sheet which approaches the kinematic split monopole solution of Bogovalov 1999 only after a careful introduction of diffusivity even in the highest resolution simulations. It also exhibits an intriguing spiral region at the crossing of two zero charge surfaces on the current sheet, which shows a destabilizing behavior more prominent in higher resolution simulations. We discuss the possibility that this region is physically (and not numerically) unstable. Finally, we present the spiral pulsar antenna radiation pattern.